In a seated position, a very small area under the buttocks supports the majority of a person's weight. In this small area, capillaries and soft tissue are compressed. Blood circulation is restricted and soft tissue is put under stress. Prolonged sitting over time can damage the tissue being compressed. The simple solution is to avoid sitting for prolonged periods, but such a solution is not realistic for many people who must sit for prolonged periods to perform many necessary functions such as driving or working.
Two major factors that contribute to the physical detriments described above are time and compressive pressure. Reducing one or both of these factors reduces the stress on the soft tissue. If the compressive pressure under the buttocks is shifted back and forth between two locations, then the duration of compressive pressure experienced at one position is reduced by half. This would allow some measure of periodical relief of the pressure points. If the compressive pressure point could be rotated between several positions over time, then the time of tissue stress at each position can be further reduced. As the number of pressure points is increased, the period of stress is reduced at each pressure point. In order to obtain the maximum number useful pressure points, the pressure points need to be evenly distributed over the entire buttocks area.
One solution to this problem is a seat that tilts in two dimensions with a pivot that is located under the center of the seat. Such a seat can continuously rotate in a circular manner, thus distributing pressure over a large number of pressure points, as shown in the motion path illustrated in FIG. 1. The problem with this method is that all pressure points are limited to only one circular path under the buttocks area. This simple motion path misses the majority of possible pressure point locations.
U.S. Pat. No. 5,976,097 to Jensen and U.S. Pat. No. 5,113,851 to Gamba both disclose a chair having a seat that is permanently tilted at a fixed angle with respect to the center of the seat. The chair seat is motor-driven to rotate this tilted fixed angle in a circular manner with respect to the center of the seat. It is important to point out that the seat does not rotate. It is the seat's tilting fixed angle that rotates around the center of the seat. The direction of this circular tilting motion remains constant and the circular tilt pattern repeats identically on each rotation. Since the seat is always tilted, the seat needs to be always in motion or a seated person will be sitting in a twisted fashion, trying to compensate for the static, tilted nature of the chair. While the purpose of the chairs described in Jensen and Gamba is to prevent sitting in a static position and thus holding the same posture for prolonged period of time, sitting in these chairs requires continuous posture adjustments. FIG. 1 illustrates a graphical plot of the circular tilted motion generated by the chairs described in Jensen and Gamba. At location 1, seat 10 is tilted backwards only, as shown in FIGS. 2A and 2B. At location 2 of FIG. 1, seat 10 is tilted to the right side only, as shown in FIGS. 3A and 3B. At location 3 of FIG. 1, seat 10 is tilted to the right and tilted forward, as illustrated in FIGS. 4A and 4B. At location 4 of FIG. 1, seat 10 is tilted forward only, as shown in FIGS. 5A and 5B. For seat 10 to be level, as shown in FIGS. 6A and 6B, seat 10 travels through a path taking it through location 5 of FIG. 1. But because the seats of Jensen and Gamba rotate at a fixed angle, they never pass through this horizontal position.
While Jensen and Gamba both address part of the problem described above, and it is desirable for a seated person to change posture and not sit in a static position for extended periods of time, it is not desirable to be forced to make continuous postural changes while seated over prolonged periods of time. Due to the fixed angle of the chairs described in Jensen and Gamba and their inability to ever become level, these seats always need to be moving, thus requiring constant posture changes for a seated person, and the seat cannot be used as a regular level chair. Also, neither Jensen nor Gamba disclose or suggest any manner in which the seat can be easily stopped, or how the seat can be stopped periodically.
U.S. Pat. No. 6,033,021 to Udo discloses a self-tilting seat that utilizes two independent, unsynchronized tilting mechanisms to generate a path from two separate motors. There is no disclosure in Udo for detecting a level position. If a level position of the seat is ever reached it is achieved randomly, and not in a repeatable manner, as the two independent tilting mechanisms are not synchronized. There is a heartfelt need for a dynamic chair having a repeatable and deterministic motion path to generate a known range of postural changes to alleviate compressive pressure at as many pressure points as possible.